Method for the recovery of titanium



April 12, 1955 J. GLASSER METHOD FOR THE RECOVERY OF TITANIUM li h 2Sheets-Skeet 1 F G- 2 Tick H(|2L v REDUCTION MOLTEN SALT BATHELECTROLYTIC nszucnou m MOLTEN SALT ncl DECOMPOSITION T0 'racl (sou'n)AND nsposmou or Ti on HOT SURFACE TI i RECOVERY TiCl T HOL TiCl fREDUCTION MOLTEN SALT BATH Till, 1

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April 12, 1955 J. GLASSER METHOD FOR THE RECOVERY OF TITANIUM- 2Sheets-Sheet 2 Filed April 19, 1951 I 1 1/ JNQH 1//// Ira E27 [:T"r/auxuv 6245852 2,706,153 Patented Apr. 12, 1955 IVETHOD FOR THERECOVERY OF TITANIUM Julian Glasser, La Grange, Ill., assignor, by mesneassignments, to Kennecott Copper Corporation, New York, N. Y.

Application April 19, 1951, Serial No. 221,804

3 Claims. (Cl. 75-84) The present invention relates to a method for therecovery of substantially pure, metallic titanium in crystalline form.

Metallic titanium has long been one of the more difficult metals torecover in substantially pure form. While the ores in this metal arequite plentiful, conventional processes for ore reducing are notadaptable to the recovery of titanium because of the inherent stabilityof the titanium oxide present in the ore and the reactivity of titaniummetal in its elemental form. While processes have been developed for therecovery of titanium in elemental form, these processes usually involvethe recovery of crystalline titanium in the form of extremely smallparticles. These particles are inherently unstable and even pyrophoric,readily oxidize in the presence of air or water, and even react withnitrogen from the air to form titanium nitrides.

An object of the present invention is to provide a method for therecovery of titanium in which the metallic titanium is deposited in theform of relatively large, ductile crystals which are relatively stableto air, water, and other reagents which ordinarily attack the finelydivided crystals of titanium recovered in previously practicalprocesses.

Another object of the present invention is to recover titanium byselective deposition of metallic titanium from a liquid phase system.

Still another object of the present invention is to provide a method forthe recovery of titanium from lower titanium halides.

I have now discovered that if the proper reaction conditions areobserved, metallic titanium can be deposited upon a heated surface froma liquid bath containing lower halides of titanium. By lower halides oftitanium I mean to include the relatively unstable halides such astitanium dichloride and titanium trichloride which result from thepartial reduction of the more stable titanium tetrachloride. The processis also applicable to the recovery of metallic titanium from titaniumdibromide and tribromide. The crystals of titanium metal which areobtained by this process are relatively large, ordinarily being well inexcess of 100 microns in diameter. Crystals of this size areconsiderably more stable to air, oxidizing gases, and other reactantsthan are crystals whose dimensions are on the order of 1 to microns.

A further description of the present invention will be made inconnection with the attached sheets of drawings in which:

Figure 1 is a diagram of the mechanism of a reaction according to thepresent invention;

Figure 2 is a diagram of another reaction mechanism which may beemployed;

Figure 3 is a graph showing the equilibrium gas pressures of titaniumtetrachloride over a titanium dichloridesodium chloride system asemployed in the process of Figure 1 at various concentrations andtemperatures; and

Figure 4 is a schematic diagram of one type of apparatus which may beemployed in the practice of the process.

In the embodiment of the process illustrated in Figure 1, l haveillustrated a reduction stage 10 in which a supply of titaniumtetrachloride and hydrogen gas are introduced into a molten salt bath.The salt used in the bath should have a melting point below about 1000C. so that the proper temperature diiferentials may be maintained withinthe process. Salts of alkali metals, and alkaline earth metals, as wellas magnesium salts may be employed for this purpose. The most commonsalt bath WhlCh may be employed consists of sodium chloride, but it willbe appreciated that other salt baths such as sodium bromide, calciumchloride, calcium fluoride, magnesium chloride, and the like maysimilarly be employed.

One of the advantages of employing a molten salt bath for the partialreduction of titanium tetrachloride to a lower halide of titaniumresides in the fact that lower temperatures can be employed where thebath is used. For example, if titanium tetrachloride is reacted withhydrogen in the absence of the salt bath, temperatures on the order of1200 C. must be employed. On the other hand, by using a molten salt bathas the vehicle for the reaction, temperatures only slightly above themelting point of the salt, for example, temperatures of about 900 C.,will be sufiicient for the reaction. A more important advantage arisingfrom the use of a salt bath for the partial reduction of titaniumtetrachloride to titanium trichloride is the fact that the reaction goesmore completely, and the recovery is quantitative, since the trichlorideis dissolved in the salt bath as soon as it forms, thus reducingpossible losses through volatilization. Another advantage of the presentprocess is the fact that the purity of the raw materials is notcritical.

The reduction of titanium tetrachloride by means of hydrogen atrelatively low temperatures and normal pressures produces both titaniumtrichloride and titanium dichloride, with the former predominatingaccording to the following equation:

2TiCl H; 2TiCl l ZHCl (Gas) (Gas) ($0111.) (Gas) The above reaction iscarried out in an air-free, nonoxidizing atmosphere, and preferably inan atmosphere of hydrogen, argon, helium, neon, or other inert gas.

In place of the hydrogen reduction of titanium tetrachloride to yieldlower halides of titanium, a suitable starting material for the processof the present invention can be prepared by partially reducing titaniumterachloride by means of other reducing agents, as, for example,metallic sodium, metallic magnesium, or metallic titanium, all of thesebeing used under a protective atmosphere of an inert gas. impuretitanium can be. used as a reducing agent, and in these circumstances,the process would be one for purifying impure titanium or its alloys torecover pure titanium.

The titanium trichloride produced in the reduction stage 10 is furtherreduced to the dichloride in a second reaction stage 11. For purposes ofconvenience, the two reduction stages it) and 11 have been illustratedin separate zones in the drawings, to illustrate more clearly themechanism of the reaction. It will be understood that both reductionreactions can be, and preferably are, carried out concurrently in aclosed system under a protective atmosphere of a non-oxidizing gas suchas hydrogen.

The titanium trichloride, dissolved in a molten salt bath maintained attemperatures above its melting point, is reduced by elemental titaniumaccording to the following equation:

The next stage of the process involves the decomposition of the titaniumdichloride produced in the previous reaction to metallic titanium byselective deposition upon a hot surface. Titanium dichloride andtitanium trichloride are soluble in each other and each is soluble inthe molten salt bath. This feature permits the deposition of metallictitanium from a purely liquid system instead of from vapor phase, as haspreviously been done. The decomposition of the titanium dichloride tometallic titanium has been indicated in Figure 1 of the drawings in thestage designated by reference numeral 12.

In the decomposition stage 12, the titanium dichloride is thermallydecomposed to yield metallic titanium according to the equation:

2TiCl Ti TiCl,

$0111.) (Solid) (Gas) A portion of the titanium which is recovered fromthe deposition is returned to the reduction stage 11 to assist in thereduction reaction, and the titanium tetrachloride produced by thedecomposition of the dichloride can be reacted with additional amountsof hydrogen to produce the trichloride as illustrated in the drawings.

In the embodiment of the invention illustrated in Figure 2 thedisproportionation of titanium trichloride to the dichloride andtetrachloride is carried out by an electrolytic reduction process. Thisprocess consists in immersing a pair of inert electrodes into the fusedsalt bath containing the titanium trichloride dissolved in the bath,whereby the trichloride undergoes a disproportionation reaction to formtitanium dichloride at the cathode and titanium tetrachloride at theanode.

In stage 13, Figure 2, the dichloride formed at the cathode isdisproportionated to titanium and titanium trichloride at a hot surfaceby virtue of keeping the trichloride concentration very low in thecatholyte. The trichloride from stage 13 is recirculated to theelectrolytic reduction stage 14 for the purpose of minimizing theconcentration of the trichloride in the catholyte.

Titanium dichloride is disproportionated to titanium trichloride andmetallic titanium in stage 13 according to the following equation:

3TiC1 ---1 Ti 2TiC1 (Soln. in NaCl) (Solid) (Soln. in N801)Thermodynamic calculations show that the above reaction will proceedonly if the concentration of the trichloride is maintained suflicientlylow at a given temperature. The following table shows the mole fractionof TiCls in equilibrium with 0.1 mole fraction of the TiClz dissolved inmolten NaCl:

Mole fraction TiCl; with 0.1 M. F. 'IiClz As the disproportionationreaction is favored by elevated temperatures, a convenient method forcarrying out the reactions in stages 14 and 13 consists in first formingthe dichloride in the electrolytic reduction stage 14 at a temperaturejust above the melting point of the salt, and subsequentlydisproportionating the dichloride produced at temperatures from 300 to500 C. above the melting point of the salt.

The concentration of TiCls in the disproportionation reaction mixtureshould be less than 0.05 mole fraction, and preferably less than 0.01for 0.1 mole fraction of TiClz. One method by which the concentration ofTiCls could be held at a minimum consists in combining stages 13 and 14,and locating the hot body upon which the titanium is to be deposited inclose proximity to the cathode. In the process of Figure 2, there is noneed for atmosphere control, in that the control comes in thecomposition of the bath.

One of the essential considerations in the practice of the processes ofFigures 1 and 2 is the maintenance of a stable salt bath. It is of primeimportance to keep the bath stable so that the thermal decomposition ofthe lower halides of titanium to metallic titanium takes place at thesurface of the hot body which is introduced into the bath instead ofwithin the bath itself. The minimum temperature of the bath will bedetermined by the melting point of the salt, While the maximumtemperature which can be employed in the bath will be limited by thetemperature differentials to be observed between the bath temperatureand the temperature of the heated body upon which the titanium isdeposited.

In Figure 3 of the drawings, there is illustrated a graph showing thecorrelation between several of the factors involved in maintaining astable fused bath of sodium chloride and titanium dichloride in theprocess of Figure 1. In the graph, the abscissae represent theactivities of titanium dichloride in a mixture of titanium dichlorideand sodium chloride. The activity coefiicient of titanium dichloride wasassumed to be unity, so that the activity will be equal to the molefraction of titanium dichloride in the system.

The ordinates of the graph represent the equilibrium pressures oftitanium tetrachloride over the titanium dichloride-sodium chloridesystem.

Another important consideration in the practice of the present processis the control of the pressure of titanium tetrachloride over thesystem. The partial pressure should be less than 10 millimeters ofmercury but more than .01 millimeter of mercury, and preferably from .2to 5 millimeters.

The reactions described in connection with Figure 1 are carried out in aclosed system, i. e., one in which the ambient pressure throughout thesystem is carefully controlled. The control of the pressure of titaniumtetrachloride over the system may be accomplishd by providing a coldtrap containing a liquid supply of titanium tetrachloride. In a largesystem, the partial pressure of the titanium tetrachloride over thesystem will closely approximate the vapor pressure of the titaniumtetrachloride at the temperature of the cold trap. Thus, for example, insuch a system the partial pressure of titanium tetrachloride can bemaintained at a value of about 0.5 millimeter by providing a supply ofliquid titanium tetrachloride at a temperature of 20 C. in communicationwith the system.

An important consideration in the practice of the processes of bothFigures 1 and 2 is the temperature differential to be maintained betweenthe surface of the hot body upon which the deposition of titanium is tobe accomplished, and the temperature of the bath. I have found that forbest results this temperature differential should be at least 100 C.,and preferably on the order of 300 C.

The body upon which the titanium is to be deposited should be arefractory substance having a melting point well above the temperatureof the bath. As an example, the heated metal body can take the form ofan electrically heated filament of tungsten, molybdenum, or titanium.Alternatively, solid rods of these metals, as well as graphite rods, maybe used as the surfaces upon which the titanium is selectivelydeposited.

It is particularly important for proper deposition in the process ofFigure 1 that the hot body he introduced into the surface of the bathvery carefully, so that the surface of the heated body is immersed onlyto a depth less than that at which the static head of the bath exceedsthe ditference between the equilibrium pressure of titaniumtetrachloride at the temperature of the hot surface and the pressure oftitanium tetrachloride above the system.

The following example illustrates the manner in which the properoperating conditions are obtained for the process of Figure 1 from thegraph of Figure 3. Forthe purposes of the example, it will be assumedthat the pressure of the titanium tetrachloride over the system ismaintained at a value of 0.5 millimeter, and that the mole fraction oftitanium dichloride in the titanium dichloridesodium chloride system is0.1. The latter composition is represented by the vertical lineextending from the point A along the abscissa of the diagram. At thisconcentration, the minimum temperature of the hot surface or the maximumtemperature of the bath can be determined by the intersection with theline from the point A of the horizontal line representing an equilbriumpressure of 0.5 millimeter. This point of intersection has been labeledas point B. The corresponding temperature is then determined byextrapolating between the parallel sloping temperature lines indicatedon the drawing, and in this instance is found to be approximately 1100"C. The shaded area of the diagram represents the tem peratures to beemployed on the heated body for deposition purposes. Theoretically, thetemperature of the heated body need be only slightly higher than thetemperature of the bath, but in practice it is impossible to control thetemperature to such a fine degree. Consequently, for best results, Ihave found that a minimum temperature differential of C. should bemaintained between the bath and the hot surface. In the cited example,the temperature for the hot surface would be at least 1200 C., andpreferably about 1300 C. with the bath below 1100 C., but above itsmelting point.

An apparatus suitable for carrying out the process of Figure 1 has beenillustrated in Figure 4. The apparatus there shown includes a reactionvessel 20 having a large iameter bulb portion 21 and a reduced diameterneck 22. The vessel is sealed from the atmosphere by means of a seal 23inserted in the end of the neck 22.

The bulb 21 contains a liquefied bath 24 of titanium subhalides inadmixture with sodium chloride. To attain the temperatures required tokeep the bath 24 molten, the bulb 21 is disposed in an electricallyheated split tube furnace 25 containing heating elements 37.

A pair of conductors 26 connected to a source of current, not shown,extend through the seal 23 and are joined at one end of a refractoryfilament 27. The depth to which the filament 27 is immersed in the bathis less than that at which the static head of the bath exceeds thedifference between the equilibrium pressure of titanium tetrachloride atthe temperature of the filament and the pressure of titaniumtetrachloride over the system, as previously explained. After the bath24 has reached the proper operating temperature, it is possible todiscontinue heating by means of the furnace 25, as the heat dissipatedfrom the filament is usually sufficient to maintain the bath molten.

The control of the partial pressure of titanium tetrachloride over thesystem is accomplished by providing a cold trap 28 in communication withthe reaction vessel 20. A liquid body of titanium tetrachloride 29 ismaintained at a suitably low temperature, normally on the order of 20C., where the vapor pressure of titanium tetrachloride approximates thedesired pressure of titanium tetrachloride over the system. Thus, astitanium tetrachloride vapors are evolved from the highly heated bath24, the vapors condense in the cold trap 28, thus effectivelymaintaining the pressure of titanium tetrachloride over the system atthe vapor pressure of titanium tetrachloride at the temperature of thecold trap.

A pump 30 is connected to the system by means of a vacuum line 31 and avalve 32 to evacuate the system in order to have only titaniumtetrachloride in the system.

A vessel of helium 33 is provided to introduce an inert gas into thesystem when the filament 27 is to be withdrawn from the vessel 20. Thehelium gas is purified by passage through a desiccant 34, and anadsorbent material such as activated charcoal 35 disposed in a bath ofliquid air 36 before being introduced into the vessel 20.

While not shown in the schematic drawing, it will be understood thatmeans will be provided in the vessel 20 to adjust the depth to which thefilament 27 is immersed within the bath 24 without affecting thepressure within the system.

To illustrate the actual operating conditions of the deposition, thefollowing example is submitted.

A bath was formed containing 20% titanium dichloride and 80% by weightsodium chloride. An electrically heated tungsten filament, was thenpositioned to a depth of about A. inch below the surface of the bath.The partial pressure of titanium tetrachloride over the system wasmaintained at 0.5 mm., and a bath temperature of. 900 C. was used. Thetemperature of the filament was in the neighborhood of 1200 C.

As soon as the deposition commenced, the apparent resistance of thetungsten filament decreased. Eventually, the current drawn by thefilament became more or less constant.

When the tungsten filament, with the elemental titanium depositedthereon was withdrawn from the bath, it was found that approximately onehalf of the titanium originally present in the bath had been convertedto metallic titanium.

The crystalline size of the titanium particles recovered was in theneighborhood of 200 microns. These crystals are ductile, stable in air,and can be leached with aqueous reagents without substantialdeterioration.

The process of the present invention is also amenable to operation in acontinuous manner. For exam le. a heated refractory metal wire may beguided through the molten bath at a predetermined distance below thelevel of the bath, and the titanium removed continuously as a deposit onthe refractory metal wire.

It will be understood that modifications and variations may be effectedwithout departing from the scope of the novel concepts of the presentinvention.

I claim as my invention:

l. In a method of recovering titanium from a system consistingessentially of a titanium dihalide dissolved in a molten alkali metalsalt bath, and a source of a liquid titanium tetrahalide, the halogen ofeach of said halides being the same and being selected from the groupconsisting of chlorine and bromine, said liquid tetrahalide being inopen communication with said bath to provide a partial pressure of saidtitanium tetrahalide over said bath, the steps comprising immersing anindependently heated refractory metal surface in said molten bath,maintaining said bath at a temperature of between its melting point and1100 C. and said refractory body at a temperature of at least C. abovethat of said bath, limiting the depth of immersion of said refractorysurface to less than that at which the static head of the bath over saidrefractory surface exceeds the difference between the equilibriumpressure of said titanium tetrahalide at the temperature of said heatedsurface and the pressure of said titanium tetrahalide over said bath,and maintaining said temperature differential to effect decomposition ofsaid titanium dihalide to titanium metal and consequent deposition ofsaid titanium metal on said heated surface.

2. In a method of recovering titanium from a system consistingessentially of titanium dichloride dissolved in a molten sodium chloridebath and a source of liquid titanium tetrachloride in open vaporcommunication with said bath to provide a partial vapor pressure oftitanium tetrachloride over said bath, the steps comprising immersing anelectrically heated refractory wire in said molten bath, maintainingsaid bath at a temperature of between its melting point and 1100" C. andsaid wire at a temperature of at least 1200 C., limiting the depth ofimmersion of said wire in said bath toless than that at which the statichead of said bath over said wire exceeds the difference between theequilibrium pressure of said titanium tetrachloride at the temperatureof said wire and the pressure of said titanium tetrachloride over saidbath, maintaining a temperature differential of at least 100 C. betweensaid wire and said bath to effect decomposition of said titaniumdichloride to titanium metal and consequent deposition of said titaniummetal on said heated wire.

3. In a method of recovering titanium from a system consistingessentially of titanium dichloride dissolved in a molten alkali metalsalt bath, and a source of liquid titanium tetrachloride, said liquidtitanium tetrachloride being in open vapor communication with said bathto provide a partial pressure of said titanium tetrachloride over saidbath, the steps comprising immersing an independently heated refractorysurface in said molten bath, maintaining said bath at a temperature ofbetween its melting point and 1100 C. and said refractory surface at atemperature at least 100 C. above that of said. bath, limiting the depthof immersion of said refractory surface to less than that at which thestatic head of the bath over said refractory surface exceeds thedifference between the equilibrium pressure of said tetrahalide at thetemperature of said heated surface and the pressure of said titaniumtetrachloride over said bath, and maintaining said temperaturedifferential to effect decomposition of said titanium dichloride totitanium metal and consequent deposition of said titanium metal on saidheated surface.

References Cited in the file of this patent UNITED STATES PATENTS723,217 Spence Mar. 17, 1903 1,046,043 Weintraub Dec. 3, 1912 1,173,012Meyer et al. Feb. 22, 1916 1,306,568 Weintraub llune 10, 1919 1,427,919Stock et a1 Sept. 5, 1922 1,861,625 Driggs et al. June 7, 1932 2,148,345Frcudenberg Feb. 21, 1939 2,178,685 Gage Nov. 7, 1939 2,205,854 KrollJune 25, 1940 2,443,253 Kroll et al. .a June 15, 1948 2551,341 Scheer etal. May 1, 1951 2,586,134 Winter et al Feb. 19, 1952 2,618,549 Glasseret al Nov. 18, 1952 OTHER REFERENCES A Comprehensive Treatise onInorganic and Theoretical Chemistry, by Mellor, vol. 7. Published 1927by Longmans, Green and Co., 55 Fifth Avenue, New York. Page 11.

Titanium Report of Symposium on Titanium. Published by Ofiice of NavalResearch, Dept. of Navy, Washington, D. C. March 1949. Page 20.

1. IN A METHOD OF RECOVERING TITANIUM FROM A SYSTEM CONSISTINGESSENTIALLY OF A TITANIUM DIHALIDE DISSOLVED IN A MOLTEN ALKALI METALSALT BATH, AND A SOURCE OF A LIQUID TITANIUM TETRAHALIDE, THE HALOGEN OFEACH OF SAID HALIDES BEING THE SAME AND BEING SELECTED FROM THE GROUPCONSISTING OF CHLORINE AND BROMINE, SAID LIQUID TETRAHALIDE BEING INOPEN COMMUNICATION WITH SAID BATH TO PROVIDE A PARTIAL PRESSURE OF SAIDTITANIUM TETRAHALIDE OVER SAID BATH, THE STEPS COMPRISING IMMERSING ANINDEPENDENTLY HEATED REFRACTORY METAL SURFACE IN SAID MOLTEN BATH,MAINTAINING SAID BATH AT A TEMPERATURE OF BETWEEN ITS MELTING POINT AND1100*C. AND SAID REFRACTORY BODY AT A TEMPERATURE OF AT LEAST 100*C.ABOVE THAT OF SAID BATH, LIMITING THE DEPTH OF IMMERSION OF SAIDREFRACTORY SURFACE TO LESS THAN THAT AT WHICH THE STATIC HEAD OF THEBATH OVER SAID REFRACTORY SURFACE EXCEEDS THE DIFFERENCE BETWEEN THEEQUILIBRIUM PRESSURE OF SAID TITANIUM TETRAHALIDE AT THE TEMPERATURE OFSAID HEATED SURFACE AND THE PRESSURE OF SAID TITANIUM TETRAHALIDE OVERSAID BATH, AND MAINTAINING SAID TEMPERATURE DIFFERENTIAL TO EFFECTDECOMPOSITION OF SAID TITANIUM DIHALIDE TO TITANIUM METAL AND CONSEQUENTDEPOSITION OF SAID TITANIUM METAL ON SAID HEATED SURFACE.